1. Field of the Invention
The present invention relates generally to a ceramic composition for dielectric ceramic materials for ceramic green sheets, press-molded shapes or pastes used for forming dielectric bodies or articles such as ceramic capacitors, thick-film capacitors and ceramic piezoelectric actuators. More particularly, the invention is concerned with a dielectric ceramic composition adapted to be suitably fired in a non-oxidizing atmosphere.
2. Discussion of the Prior Art
In a known method of fabrication of laminar ceramic capacitors or piezoelectric actuators, electrodes or other conductive members are printed on a dielectric ceramic green sheet, by using a conductor paste whose major component consists of a noble metal such as platinum or palladium that is stable in an oxidizing atmosphere. A plurality of such dielectric green sheets with conductive members formed thereon are superposed on each other, and the obtained stack of green sheets is fired in an oxidizing atmosphere. Suitable leads are formed on the fired ceramic body, for electrical connection of the electrodes to external elements.
The above known method is advantageous in that the ceramic material can be fired in air. However, the same method is disadvantageous from the standpoint of economy, since the method uses platinum, palladium or other expensive noble metals for the conductive members of the capacitors or actuators.
To solve the drawbacks of the above known method, it is considered to use a conductor paste whose major component consists of nickel, copper, molybdenum or other non-noble or base metals. This conductor paste is applied to suitable dielectric ceramic green sheets, and a stack of these green sheets is fired in a non-oxidizing atmosphere, whereby the desired dielectric cermaic article is prepared. This alternative method also suffers from inconveniences, which arise from the firing operation effected in a non-oxidizing atmosphere. For example, a compound having a perovskite structure or a complex perovskite structure included in the dielectric ceramic composition tends to be reduced or denatured, or oxides or compounds of bismuth, zinc, lead or other elements used as additives may be evaporated, during the firing operation. Therefore, the composition of the dielectric ceramic material is changed during the firing process in a non-oxidizing atmosphere. Thus, the alternative method does not permit the fabricated dielectric ceramic article to have a sufficiently dense ceramic structure. The dielectric ceramics produced according to the alternative method in question is likely to have a low dielectric constant and a high dielectric loss tangent (high dielectric dissipation factor).